1. Field of the Invention
The present invention relates to a developing apparatus to be used for an image forming apparatus using an electrophotographic system or an electrostatic recording system, and more particularly to a developing apparatus using two-component developer including carrier and toner.
2. Related Background Art
Conventionally, various types of image forming apparatuses of the electropotographic type have been proposed and put to practical use. The image forming apparatus has developed an electrostatic latent image formed on an image bearing body by a developing apparatus for visualization, and this developing is generally divided into a single-component developing method and a two-component developing method.
Most of the single-component developing method uses the non-contact system, and as a typical developing method of the single-component developing method, there is a single-component jumping developing method using magnetic toner. This developing method is capable of obtaining a high quality image in an easy developing apparatus configuration. But it has a disadvantage that any color image cannot be obtained since toner contains magnetic substance. On the other hand, the single-component developing method using non-magnetic toner is capable of obtaining a color image, but it is difficult to apply toner to a developing sleeve of the developing apparatus. It is currently coated through the use of an elastic blade, and this method lacks stability and durability.
In contrast, the two-component developing method is to convey toner to a developing unit through the use of a magnetic carrier for developing, and the developer is normally caused to come into contact with a photosensitive drum for developing. With reference to FIG. 5, the description will be made of the developing process.
As shown in FIG. 5, the developing apparatus is configured by providing a developing container 34 containing developer with a developing sleeve 30 comprising a magnet roller 35 disposed so as not to rotate, conveying screws 31 and 32, and a regulating blade 33. The description will be made of a process for developing an electrostatic latent image on the photosensitive drum 3 in accordance with the two-component magnetic brush method through the use of this developing apparatus, and circulation of the developer at the time hereinafter.
First, the developer, which has been conveyed through the use of the conveying screws 32 and 31 and attracted on the surface of the developing sleeve 3 by a magnetic pole N3 of the magnet roller 35, is conveyed to N3 polexe2x86x92S2 polexe2x86x92N1 pole with the rotation of the developing sleeve 30, is regulated by the regulating blade 33 in the process, and is formed into a thin layer of developer on the developing sleeve 30. When the developer formed into the thin layer is conveyed to a developing main pole S1 of a magnet roller 35 located in a developing unit in which the photosensitive drum 3 and the developing sleeve 30 are opposed to each other, a magnetic force causes the developer to stand on the surface of the developing sleeve 30 like ear, and the developer formed into an ear shape comes into contact with the surface of the photosensitive drum 3, and develops the electrostatic latent image on the photosensitive drum 3 to visualize it as a toner image.
The developer after the development is completed in the developing unit is returned into a developing container 34 with the rotation of the developing sleeve 30, and is separated from the developing sleeve 30 by means of a repulsion magnetic field to be formed by magnetic poles N3 and N2 of the magnet roller 35 to be collected within the developing container 34.
As described above, in the two-component developing method, the structure is generally arranged such that magnetic poles having the same polarity are arranged side by side, and developer after development is removed from the developing sleeve once so as not to leave any image history on the developing sleeve.
As a method for forming an electrostatic latent image, there has been known a method for exposing and scanning the surface of the photosensitive drum 3 through the use of a laser beam modulated correspondingly to an image signal to be recorded to form into a dot-shaped latent image obtained by bringing a shape of distributed dots into correspondence with an image. Of these methods, a so-called pulse width modulation (PWM) method for modulating a width (that is, duration) of laser driving pulse current correspondingly to density of a non-recorded image is capable of obtaining high recording density (that is, high resolution), and obtaining high gradation.
In recent years, in an image forming apparatus using the two-component developing apparatus, further miniaturization, higher image quality and extension of the life have been advanced. Of these, in order to attain extension of the life of the developing apparatus, it is necessary to arrange the structure such that the developer is not compressed and to prevent toner and carrier from being deteriorated.
A place where the developer is compressed within the developing container 34 is a developer regulating unit, and according to an ordinary configuration of the developing apparatus, a developer layer thickness regulation pole S2 of the magnet roller 30 is positioned upstream of the regulating blade 30 in the direction of rotation of the developing sleeve 30, and the developer attracted to the regulation pole S2 is compressed between the developing sleeve 30 and the container 34 in this domain.
In order to weaken the compression of developer, it is effective to weaken a force, magnetic attraction force working in a direction perpendicular to the surface of the developing sleeve 30, by which the developer layer thickness regulation pole S2 is attracting the developer to the developing sleeve 30.
As a method for weaking the magnetic attraction force, there are a method for reducing magnetization of magnetic carrier in the developer, or constructing such a magnet pattern as to appear perpendicularly from the surface of the developing sleeve 30 as far as possible and so on because it is difficult for a line of magnetic force from the developer layer thickness regulation pole S2 to go round to magnetic poles N3 and N1 adjacent. In the above method for reducing magnetization of magnetic carrier a direction for reducing this carrier magnetization is an advantageous direction to improve the image quality at a point where a force for slidably contacting the toner image developed on the photosensitive drum in the developing unit becomes weak.
As one of the latter methods using a magnet pattern, there has been proposed a developing method for using one of repulsion magnetic fields of developing sleeve as the developer layer thickness regulation pole (Japanese Patent Application Laid-Open No. 11-311904). When magnetic poles having the same polarity are adjacent to each other to form a repulsion magnetic field, the line of magnetic force from each magnetic pole emanates perpendicularly to the surface of the developing sleeve, and a rate of change in magnetic flux density in a direction perpendicular to the surface of the developing sleeve is small. As a result, a force which attracts the developer to the developing sleeve becomes small to weaken the compression of the developer.
However, when the structure is arranged such that one of such repulsion poles as described above is used as a developer layer thickness regulation pole, screw pitch-shaped density unevenness occurred at the rear end of a solid black image.
This phenomenon occurs when a mixture ratio between developer having an image history after development which could not be removed by the repulsion magnetic field and developer supplied to the developer layer thickness regulation pole S2 after agitated and conveyed by a conveying screw 31 is changed by a rotational period of the screw in an image longitudinal domain. Also, this phenomenon also easily occurs when the magnetic carrier is less magnetized. This is because when less magnetized, the developer becomes insensitive to the magnetic field, and the developer after the development cannot be removed with a scraping pole, but is prone to move to the developer layer thickness regulation pole S2.
It is an object of the present invention to provide a developing apparatus using one of magnetic poles for forming the repulsion magnetic field as a developer amount regulation pole.
It is another object of the present invention to provide a developing apparatus which is not affected by a pitch of an agitating screw.
It is still another object of the present invention to provide a developing apparatus, including:
a developing container containing developer including magnetic carrier and non-magnetic toner;
a developer bearing body provided at an opening of the developer container, for rotating while bearing the developer;
a magnet member provided within the developer bearing body, the magnet member having a first magnetic pole, and a second magnetic pole adjacent on the downstream side of the developer bearing body of the first magnetic pole in a direction of rotation thereof, having a second magnetic pole having the same polarity as the first magnetic pole; and
a regulating member provided in the vicinity of the second magnetic pole, for regulating an amount of developer on the developer bearing body,
in which between the first magnetic pole and the second magnetic pole, there exists 40xc2x0 or more a domain having 8 mT or less in magnetic flux density in the direction of the normal to the surface of the developer bearing body.
Further objects of the present invention will be apparent from the following description.